1. Field of the Invention
The present invention relates to magnetic sensors, each of which includes a plurality of giant magnetoresistive elements formed on a single substrate so as to detect magnetic field intensities (or magnetic field strengths) in two-axial directions or three-axial directions. The present invention also relates to manufacturing methods of magnetic sensors.
The present application claims priority on Japanese Patent Application No. 2007-110475, the content of which is incorporated herein by reference.
2. Description of the Related Art
As elements used for magnetic sensors, giant magnetoresistive elements (GMR elements) and tunnel magnetoresistive elements (TMR elements) have been conventionally known. Each of the conventionally-known magnetoresistive elements includes a pin layer whose magnetization direction is pinned (or fixed) and a free layer whose magnetization direction varies in response to an external magnetic field, wherein it shows a resistance to suit the mutual relationship between the magnetization direction of the pin layer and the magnetization direction of the free layer. Magnetic sensors using magnetoresistive elements have been disclosed in various documents such as Patent Document 1 and Patent Document 2.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-260064.    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-261400.
Patent Document 1 teaches a magnetic sensor that is constituted using a plurality of giant magnetoresistive elements each having a synthetic antiferromagnetic (SAF) structure, in which a Ru layer (where Ru stands for ruthenium) is inserted into a magnetic layer of a pin layer formed on the surface of a substrate. Each of the giant magnetoresistive elements detects a magnetic field intensity lying in one direction on the surface of the substrate. A plurality of giant magnetoresistive elements, which detect magnetic field intensities in different directions, is formed on the surface of the substrate. This makes it possible for the magnetic sensor to detect magnetic field intensities in two-axis directions.
In magnetization heat treatment (or pinning) for fixing magnetization directions of pin layers during manufacturing of magnetic sensors, substrates are each heated up to a prescribed temperature while a magnet away is positioned opposite to the surface of the substrate, wherein an intense magnetic field (whose value is 10 T or more, for example) is applied to the pin layer so as to allow two magnetic layers to be positioned in parallel with each other. Herein, the magnetic field applied to the pin layer occurs in a slit (e.g., a rectangular through-hole) of a yoke forming the magnet array, wherein the magnetization direction of the pin layer depends upon polarities of permanent magnets adjoining together in the magnet array. After heat treatment, the magnetic sensor is produced such that the magnetization directions of two magnetic layers are directed opposite to each other, in other words, two magnetic layers are placed in an anti-parallel state of magnetization.
Patent Document 2 teaches a small-size magnetic sensor in which planar surfaces and slopes (i.e., inclined surfaces inclined to planar surfaces) are formed on the surface of a single substrate, wherein giant magnetoresistive elements are formed on the planar surface and the slopes respectively, thus making it possible to detect magnetic field intensities in three-axial directions.
The slopes are formed by way of V-shaped channels formed on the surface of the substrate and are thus positioned opposite to each other in each channel. This magnetic sensor is formed using giant magnetoresistive elements, in which Ru layers are not inserted into magnetic layers of pin layers.
The magnetic sensor of Patent Document 2 is capable of detecting magnetic field intensities in three-axial directions, wherein magnetization directions of pin layers of giant magnetoresistive elements are affected and varied due to an excessively intense external magnetic field. When the external magnetic field becomes zero in intensity, magnetization directions are varied and fixed in wrong directions. That is, this magnetic sensor may suffer from a relatively weak resistance in ferromagnetism.
The giant magnetoresistive elements of synthetic antiferromagnetic structures used in the magnetic sensor of Patent Document 1 are each designed such that two magnetic layers forming a pin layer are placed in an anti-parallel state of magnetization, which may increase the resistance in ferromagnetism but which cannot detect magnetic field intensities in three-axial directions. For this reason, the inventor of this application modifies the magnetic sensor of Patent Document 2 such that giant magnetoresistive elements of synthetic antiferromagnetic structures are formed on planar surfaces and slopes of a substrate.
Suppose that three giant magnetoresistive elements having pin layers of different magnetization directions are formed on a planar surface and two slopes (i.e., paired slopes positioned opposite to each other via a channel) respectively. In magnetization heat treatment for fixing magnetization directions of pin layers, the overall area projecting the paired slopes having giant magnetoresistive elements whose pin layers are placed under a magnetic field becomes larger than the other area projecting the planar surface having a giant magnetoresistive element whose pin layer is also placed under the magnetic field. Herein, the width dimensions of giant magnetoresistive elements formed on the opposite slopes become larger than the width dimensions of a giant magnetoresistive element formed on the planar surface.
As described above, even when giant magnetoresistive elements of synthetic antiferromagnetic structures are applied to the magnetic sensor of Patent Document 2, it is necessary to apply an intense magnetic field whose value is 10 T or more to pin layers of giant magnetoresistive elements during heat treatment; hence, it is necessary to use a magnet array having slits. Herein, the width dimensions of a slit for applying a magnetic field to giant magnetoresistive elements formed on slopes are increased by differences of width dimensions between the slopes and the planar surface in comparison with the width dimensions of another slit for applying a magnetic field to a giant magnetoresistive element formed on the planar surface.
This may weaken the magnetic field applied to giant magnetoresistive elements formed on the slopes; hence, it becomes very difficult to apply an intense magnetic field to all the giant magnetoresistive elements during heat treatment. When a magnetic field having an inadequate intensity is applied to giant magnetoresistive elements in heat treatment, it becomes very difficult to place two magnetic layers in an anti-parallel state of magnetization after heat treatment.
In heat treatment in which an intense magnetic field is applied to giant magnetoresistive elements, variations of the intensity of the magnetic field applied to giant magnetoresistive elements become very large relative to variations of the distance between the substrate and the magnet array, which are positioned opposite to each other with a prescribed gap therebetween. That is, it is necessary to adjust the relative positioning and the distance between the substrate and the magnet array with very high precision. This may cause trouble in the manufacturing of magnetic sensors.